Ion generators are now well known, and it is likewise well known that such ion generators are normally caused to produce both positive and negative ions.
Such ion generators are used to eliminate, or at least reduce, static electricity that might be present at a preselected, or work, area. At the present time, ion generators constitute the only known practical devices for reducing the static charges acquired by tribo electric, induced, or direct charge accumulation on nonconductive materials, such as various forms of plastic.
The general principle utilized is to generate equal and sufficient amounts of both positive and negative air ions, and then propel these ions into a preselected, or work, area to thereby discharge any charged particles in the area. An equal number of positive and negative ions in the air results in an overall net charge of zero. These charges of opposite polarity can co-exist with neutral air molecules since there are about 3.times.10.sup.19 neutral air molecules for each ion, and the neutral air molecules therefore tend to isolate the charged air molecules. The oppositely charged ions neutralize each other when they meet, however, and a constant source of such ions must therefore be made available through ion generator devices. This recombination process is also responsible for neutralizing the static charges on isolated surfaces of non-conductive and conductive materials.
At the present time, there are two arrangements primarily used to produce charged positive and negative ions. One of these arrangements uses radioactive materials such as polonium to emit alpha particles that bombard neutral air molecules and produce equal numbers of positive and negative charged ions. Ion balance is not normally a problem with this type of system, since the oppositely charged molecules are created in pairs.
This system, while being relatively safe, has, however, several disadvantages including relatively low ion production, short half-life of the radioactive isotope, and stringent government control guidelines. Also, ion generation is reduced substantially over time due to the characteristics of the radioactive material, and alpha particles (which cannot penetrate the skin under normal conditions and therefore are used with this type of ionizer) are also impeded by water vapor from environments with high relative humidity conditions or which have an accumulation of smoke or dust particles surrounding the radioactive material.
The second type of electrical devices now used to generate appropriately charged ions utilize corona discharge of a high power supply fed into a sharp needle point which intensifies the field surrounding the needle. The dielectric strength of air is overcome, corona discharge occurs, and current flows either into the needle point from the air for positive ions or from the needle point into the air for negative ion generation.
Since it is generally easier to produce negative ions than positive ions, known electrical means of ion generation have, however, the disadvantage of producing an imbalance of ion charges. This can create a problem since the ion generation equipment used to discharge static charges toward a work area can also be a potentially dangerous source of problems should such equipment become unbalanced, or should a catastrophic failure occur in either the positive or negative ion generation system of the equipment.
Static charge control devices having needle electrodes for producing positive and negative ions are shown, for example, in U.S. Pat. Nos. 4,319,302 and 4,333,123 (Moulden) and in U.S. Pat. No. 3,624,448 (Saurenman). While not always necessary, these patents also illustrate the use of a forced air unit, such as a fan, to propel the ions away from the area where such ions have been generated (U.S. Pat. No. 4,038,383 to Breton, for example, shows an ion generator system wherein forced air is not utilized to propel the ions). Balancing of ions is suggested in U.S. Pat. No. 4,092,543 to Levy. In addition, applicant's U.S. patent application Ser. No. 884,009 filed July 10, 1986 and entitled "Static Charge Control Device Having Laminer Flow" and U.S. patent application Ser. No. 884,011, filed July 10, 1986 and entitled "Static Charge Control Device With Electrostatic Focusing Arrangement", teach balanced production of positive and negative ions and directing of such ions toward a preselected area for elimination of static charges thereat.
While systems and devices have heretofore been suggested to measure ion balance, devices now utilized have been relatively burdensome, complex, and/or expensive. Among such devices are ion grid meters and electrostatic field meters with mechanically movable transducers such as vibrating plates, motor driven plates, or Piezo electro-elements. In addition, inexpensive passively coupled meters have also been heretofore utilized for gross measurements, but such meters have not proved to be sufficiently sensitive and cannot continuously monitor ion balance to meet industry requirements for continuous finer measurements as the size and complexity of integrated circuits continues to decrease and increase, respectively.
With respect to ion grid meters, these devices are used to measure the number of charged molecules per unit area and rely on the principle that ionized air molecules, either positive or negative, reduce the impedance of the air pathway, and thereby will provide a lower impedance to the flow of current through the pathway. These devices require the use of very stable high voltage power supplies and a very low scale current meter, usually a nanoamp meter. By reversing the polarity of the high voltage power supply, both the number of positively and negatively charged air molecules can be measured. Any imbalance in the positive to negative charges will be directly related to the current flow through the system and therefore imbalance can be measured.
With respect to static field meters used to measure ion imbalance, since an electrostatic field, by definition, does not move, such a device must have an element which does move within the field in order to measure the effect. Therefore, the transducer pickup device usually incorporates some mechanical means to move a plate or conductor in the vicinity of the electrostatic field. These devices are usually small plates vibrating at precise frequencies and distances, which cut through the static field and are converted into field intensity measurements on some form of visual indicator. Small motors that move one plate in relation to another conductive plate or Piezo-electric elements that vibrate at precise frequencies are also used.
With respect to inexpensive high voltage meters that have been developed to measure discharge rates of air ionization equipment, such devices require an isolated plate isolated some distance from the ionizer to be charged at a high voltage (usually about 5000 volts), and will discharge due to the ionization equipment over a period of time to a stable low voltage. This system could be used to indirectly determine ion balance if the end discharge point of the material charged is known. Most non-conductive materials will not fully discharge to zero, however, due to the internal physics of the material itself and its environment.
As can be appreciated from the foregoing, a relatively simple and compact device for monitoring of ion balance can be utilized to good advantage to detect ion balance at a preselected area.